It is generally known to use Coriolis effect mass flow meters to measure mass flow and other information for materials flowing through a conduit in the flow meter. Exemplary Coriolis flow meters are disclosed in U.S. Pat. Nos. 4,109,524, 4,491,025, and Re. 31,450 all to J. E. Smith et al. These flow meters have one or more conduits of straight or curved configuration. Each conduit configuration in a Coriolis mass flow meter has a set of natural vibration modes, which may be of simple bending, torsional, or coupled type. Each conduit can be driven to oscillate at resonance in one of these natural modes. Material flows into the flow meter from a connected pipeline on the inlet side of the flow meter, is directed through the conduit or conduits, and exits the flow meter through the outlet side of the flow meter. The natural vibration modes of the vibrating, material filled system are defined in part by the combined mass of the conduits and the material flowing within the conduits.
When there is no flow through the flow meter, all points along the conduit oscillate due to an applied driver force with identical phase or small initial fixed phase offset which can be corrected. As material begins to flow through the flow meter, Coriolis forces cause each point along the conduit to have a different phase. For example, the phase at the inlet end of the flow meter lags the driver, while the phase at the outlet leads the driver. Pick-off sensors on the conduit(s) produce sinusoidal signals representative of the motion of the conduit(s). Signals output from the pick-off sensors are processed to determine the phase difference between the pick-off sensors. The phase difference between the two or more pick-off sensors is proportional to the mass flow rate of material through the conduit(s).
Coriolis mass flow meters have received great success in a wide variety of industries. However, Coriolis flow meters along with most other flow meters can suffer from an accumulation of deposits left by the process fluid. This accumulation is generally referred to in the art as “coating.” Depending on the characteristics of the process fluid, the fluid coating may or may not affect the flow meter's performance and accuracy. Although the coating generally will not affect the flow meter's stiffness nor cause a flow rate measurement error, it can affect other aspects of the flow meter's characteristics. For example, the coating may have a different density than the process fluid. This can adversely affect the density reading obtained from the flow meter. With certain process fluids, the coating may build up inside the flow meter to a certain thickness and then break off as small flakes. These small flakes may affect other parts of the process connected to the flow meter. In extreme circumstances, the coating may build up enough such that the flow meter becomes plugged requiring complete shut down or in some circumstances, a complete replacement of the flow meter.
Other problems may be caused by coating, plugging, inconsistent process fluid compositions, changes in temperature of the process fluid, etc. For example, in the paint industry, the same flow meter may be used for multiple paint colors. Therefore, even though the coating may not cause meter reading errors, the coating could adversely affect the end product.
Because of the above problems, along with others caused by coating, it is desirable to diagnose when there is flow meter coating. Prior art diagnostic methods of detecting flow meter coating have a number of problems. First, many of the prior art methods are limited to coating detection in the active section of the flow tube, i.e., the vibrating section. Other limitations of the prior art arise in situations where the density of the coating is substantially similar to the process fluid. In those circumstances, density based coating detection is not available. Therefore, there is a need in the art for a coating detection method that overcomes the above mentioned limitations. Furthermore, in applications where it is known that the process fluid coats the flow meter, it is desirable during cleaning of the flow meter to be able to detect when the meter is completely uncoated.